1. Field of the Invention
The present invention relates to an optical lens to be mounted in an optical information recording and reproducing apparatus capable of executing at least one of the recording of information to an optical recording medium and the reproducing of the information recorded on the optical recording medium. The present invention also relates to the optical information recording and reproducing apparatus equipped with the optical lens.
2. Description of Related Art
According to an earlier development there has been an optical pickup apparatus (an optical information recording and reproducing apparatus) executing the recording of information to an optical recording medium and the reproducing of the information recorded on the optical recording medium. In the optical pickup apparatus, light emitted from a semiconductor laser light source is condensed on the information recording surface of an optical recording medium by an object lens (an optical lens) to perform the recording and the reproducing of the information.
In the object lens, an anti-reflection coat is formed on the surface of the object lens in order to improve the usability of light. The anti-reflection coat is generally formed to have a thickness which becomes thinner as a position in a peripheral part becomes more distant from the central part of the object lens. The film thickness is set in order that reflectance of the light entering the central part of the object lens perpendicularly may have a local minimum value to the wavelength of the laser beam of the optical pickup apparatus. That is, the film thickness of the anti-reflection coat is set in order that the quantity of the light which is transmitted through the central part of the object lens may be the maximum.
Now, in the anti-reflection coat, it is known that the larger the incident angle of light is, the more the wavelength dependency of the reflectance of the light shifts to a short wavelength side. Moreover, in the object lens, the more a position of the object lens is deviated to the peripheral part from the central part of the lens, the larger the incident angle of light is. Consequently, in an object lens according to an earlier development, the wavelength dependency of the reflectance of the light at the peripheral part was shifted to the shorter wavelength side more than the wavelength dependency of the reflectance of the light at the central part. As a result, the wavelength at which the reflectance of the light entering the peripheral part had a local minimum value was shorter than the wavelength at which the reflectance of the light entering the central part had a local minimum value. Hence, in the object lens provided with an anti-reflection coat according to an earlier development, the reflectance to a laser beam was higher at a peripheral part although the reflectance to the laser beam was low at the central part. Consequently, the quantity of the light which is transmitted through the peripheral part is relatively less than the quantity of the light which is transmitted through the central part. As a result, there were problems of the deterioration of the spectral intensity of the transmitted light of the whole lens, the increase in the diameter of a beam spot owing to the degradation of the focusing performance of a beam, the lowering of a beam quantity, and the like.
Moreover, in recent years, because of the increase in the capacity of the optical recording medium, it has been attempted to make the diameter of a beam spot small, that is, to stop down the beam spot sufficiently with the object lens in order to make it possible to perform the recording and the reproducing of information by the use of light in a state of high density recording. Since the diameter of a beam spot is inversely proportional to the numerical aperture (NA) of the object lens, the NA of the object lens has been heightened. Lately, a lens having an angle, which is formed by the crossing of a normal line of a lens effective diameter surface and an optical axis, of more than 45 degrees or more, or further more than 55 degrees or more has been used.
However, since the curvature of the lens surface of an object lens having a high NA is large, the incident angle of the light entering the peripheral part of the lens is very large. Consequently, the quantity of the light which is transmitted through the peripheral part is largely decreased, and it is impossible to suppress the increase in the spot diameter despite the usage of the high NA object lens. Therefore, the increase in the capacity of the optical recording medium was prevented.
Accordingly, as a technique for solving the problems described above, there is a technique for increasing the transmitted light quantity of the whole lens by increasing the transmitted light quantity at the peripheral part. The technique is disclosed in, for example, Japanese Patent Laid-Open Publication No. Hei 10-160906, Japanese Patent Laid-Open Publication No. Hei 11-222446 and Japanese Patent Laid-Open Publication No. 2001-6204.
However, when the transmitted light quantity at the peripheral part is simply increased in accordance with the Patent Documents mentioned above, the problems of the losing of the shape of a beam, the deterioration of the jitter characteristic, the increase in crosstalk and the like were produced, and there was the possibility of causing the degradation of the performance of recording and reproducing. Thus, it was difficult to optimize the balance between the light condensing performance and the light quantity.
In particular, in a Digital Versatile Disc (DVD) optical pickup lens using laser beams of two kinds of wavelengths (for example, the light having the wavelength of 660 nm and the light having the wavelength of 785 nm), it was very difficult to increase the transmitted light quantities and at the same time to optimize the beam shapes of both of the lights of the two kinds of wavelengths.